The invention relates to an apparatus for hardening a layer applied to a substrate.
The invention is concerned with the treatment of substances, more especially of dyestuffs, which mainly have a double bond as the monomer, which double bond is to be polymerised by means of UV light. It is known to polymerise double bonds with electrons or cationically.
To enable the UV radiation to start the photo reaction, a so-called photo initiator is needed in the mixture to be treated, e.g. of a dyestuff. This photo initiator is used to excess so that the polymerisation reaction, once started by UV light, is not interrupted by the dyestuff radical reacting with the diradical oxygen. Accordingly, until now, a relatively high concentration of photo initiator is used, so that the likelihood of the dyestuff radical encountering an oxygen radical and being penetrated thereby becomes minimal compared with the likelihood of its encountering another monomer with a double bond and radicalising such monomer.
Central cylinder machines are known with which various dyes are applied one after the other to a paper web or to a plastics material film, each layer being dried before the next dye layer is applied. UV emitters are used, and cooled by means of air, to dry these individual dye layers. For this, a UV lamp with an external temperature of approx. 800.degree. C. is cooled by the induction of air which is conducted past the lamp The disadvantages in this arrangement are the constant production of ozone, the movement of large quantities of dirt particles and the heating of the coated substrate which, especially with heat-sensitive plastics material films, can lead to serious imperfections.
Alterations to the known cooling system by water-cooling around or in front of the UV lamp lead to performance losses. Meanwhile, arrangements with a water-cooled housing and reflector and possibly also with a water-cooled counter-pressure cylinder are being used successfully. This structural arrangement is indeed usable in heat technology; no dirt particles are moved and no ozone is produced, but large performance losses are to be expected with a water-cooling system which encases the UV lamp.
Since, in fact, the photo initiators have the disadvantage of, on the one hand, having a relatively strong inherent smell arid, on the other hand, being very expensive, the technical problem of the invention resides in producing an apparatus in which the quantity of photo initiators can be considerably reduced.
According to the invention, this problem is solved with an apparatus according to the claims.